Measurements of volatile organic compounds over West AfricaJ. G. Murphy1,*, D. E. Oram1, and C. E. Reeves11School of Environmental Sciences, University of East Anglia, Norwich, UK*now at: Department of Chemistry, University of Toronto, Toronto, Canada

Abstract. In this paper we describe measurements of volatile organic compounds (VOC)
made using a Proton Transfer Reaction Mass Spectrometer (PTR-MS) aboard the
UK Facility for Atmospheric Airborne Measurements during the African
Monsoon Multidisciplinary Analyses (AMMA) campaign. Observations were made
during approximately 85 h of flying time between 17 July and 17 August 2006,
above an area between 4° N and 18° N and 3° W and
4° E, encompassing ocean, mosaic forest, and the Sahel desert. High time
resolution observations of counts at mass to charge (m/z) ratios of 42, 59,
69, 71, and 79 were used to calculate mixing ratios of acetonitrile,
acetone, isoprene, the sum of methyl vinyl ketone and methacrolein, and
benzene respectively using laboratory-derived humidity-dependent calibration
factors. Strong spatial associations between vegetation and isoprene and its
oxidation products were observed in the boundary layer, consistent with
biogenic emissions followed by rapid atmospheric oxidation. Acetonitrile,
benzene, and acetone were all enhanced in airmasses which had been heavily
influenced by biomass burning. Benzene and acetone were also elevated in
airmasses with urban influence from cities such as Lagos, Cotonou, and
Niamey. The observations provide evidence that both deep convection and
mixing associated with fair-weather cumulus were responsible for vertical
redistribution of VOC emitted from the surface. Profiles over the ocean
showed a depletion of acetone in the marine boundary layer, but no
significant decrease for acetonitrile.